1. Field of the Invention
The present invention relates to a switching circuit. More particularly, the present invention relates to a switching circuit for power converters.
2. Description of Related Art
FIG. 1 shows a traditional switching circuit. When the power converter is turned on, a voltage VCC is supplied from a high voltage source VIN to a control circuit 10 of the power converter via a transistor 11. Once the control circuit 10 starts to operate, it is powered by a transformer winding 100 via a diode 90 and a capacitor 70. After that, the transistor 11 will be turned off for saving power consumption. The transistor 12 is used to control the on/off operation of the transistor 11. A resistor 15 is used to provide a bias voltage to turn on the transistor 11. A control signal SN Controls the transistor 12 via an inverter 14. As the control signal SN is disabled, the transistor 12 is turned on to switch off the transistor 11. However, when the transistor 12 is turned on, the resistor 15 will consume a power PR, which is given by,
                              P          R                =                              V            IN            2                                R            15                                              (        1        )            
where R15 is a resistance of the resistor 15.
A high input voltage VIN is generally supplied from an AC line voltage. Via the rectification, the voltage of the input voltage VIN would be approximately as high as 350VDC when AC line voltage is high, such as 264VAC. Therefore, a significant power loss will be resulted from the resistor 15. The resistor 15 with mega ohms in resistance can reduce the power loss. However such high resistance resistor is not suitable to be built into an integrated circuit. Therefore, it is desirable to develop a higher efficiency switching circuit, especially for an integrated circuit for high voltage switching.